The Crystal Structure of Bixbyite and the C-Modification of the Sesquioxides

Abstract

It was discovered in 1925 by Goldschmidt that an extensive series of sesquioxides form cubic crystals with the unit of structure containing 16 M_2O_3, the value of a varying between 9.3 Å and 10.9 Å. An atomic arrangement based on the space group T^5 was assigned this C-modification of the sesquioxides by Zachariasen, who studied crystals of Sc_2O_3, Mn_2O_3, Y_2O_3, In_2O_3, Tl_2O_3, Sm_2O_3, Eu_2O_3, Gd_2O_3, Tb_2O_3, Dy_2O_3, Ho_2O_3, Er_2O_3, Tm_2O_3, Yb_2O_3, Lu_2O_3, and the mineral bixbyite, (Fe, Mn)_2O_3. Zachariasen's procedure was the following. Using the data from powder and Laue photographs of Tl_2O_3, and neglecting the contribution of the oxygen atoms to the reflections, he decided that the space group is T^5, with the 32 Tl in 8b with parameter t = 0.25, in 12c with parameter u = 0.021, and in 12c with parameter v = 0.542. These parameter values were assumed to hold for all members of the series. The consideration of intensities of reflection of Sc_2O_3 then was found to indicate the 48 O to be in two groups of 24 in the general position of T^5, with parameters x_1 ~ 1/8, y_1 ~ 1/8, x_1 ~ 3/8 and x_2 ~ 1/8, y_2 ~ 3/8, x_2 ~ 3/8. The same structure was also assigned bixbyite, with 16(Mn, Fe)_2O_3 in a unit 9.35 ± 0.02 Å on edge.

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